Teenage debutant Marc Guiu snatched champions Barcelona a 1-0 win over Athletic Bilbao just seconds after coming on from the bench on Sunday in La Liga, taking the Catalans up to third.
Xavi's Barca will face leaders Real Madrid next Saturday in the Clasico just one point behind their bitter rivals, and second-placed Girona, who beat Almeria 5-2 earlier on.
For a long time it seemed like Barcelona would not find the breakthrough against their Basque visitors but striker Guiu, 17, netted under a minute after being thrown on by Xavi.
The forward was slipped in by the silky Joao Felix and slotted past Unai Simon, who got a hand to his shot but could not keep it out.
Guiu became the latest on a conveyor belt of youths to make their debuts for Barca as their La Masia academy continues to produce gems.
"He's been training with us for the whole season, it's important that everone who comes brings something," Barcelona defender Inigo Martinez told DAZN.
"We're happy for him, for the goal, and above all, for winning."
Xavi started Martinez against his former club and 20-year-old Fermin Lopez on the left of the attack, another academy product, with Barcelona down to the bare bones.
Several regular starters, including key striker Robert Lewandowski, are out injured.
Felix clipped the top of the crossbar with a shot from Ferran Torres' cut-back as Barcelona looked to get started quickly ahead of a tricky week.
The Catalans host Shakhtar Donetsk in the Champions League on Wednesday before welcoming rivals Madrid to Montjuic.
However the hosts were negated by former Barcelona coach Ernesto Valverde's Athletic, who took the sting out of the game and occasionally sprang forward with both Williams brothers threatening from the flanks.
Marc-Andre ter Stegen tipped over an Inaki Williams effort and then parried a strike from the Ghana international, with Martinez producing a sublime block to deny his sibling Nico from the rebound.
Approaching the hour mark Barcelona carved out their best chance, when Simon brilliantly denied Lopez from close range, after initially saving from Felix's low strike.
Another followed when Felix fed substitute Lamine Yamal in the area but the 16-year-old starlet dragged his shot wide of the near post.
The teenager's arrival helped Barcelona gain the upper hand and Joao Cancelo was the next to draw a save from the watchful Simon.
- Instant impact -
Xavi brought on Guiu in the 79th minute and with his very first touch he broke into the area and then with another confidently fired past Simon.
Full of energy, the striker was soon booked for an attempted slide tackle which caught the goalkeeper as he tried to stop him clearing his lines, as he was roared on by the jubilant home fans.
Barcelona's victory takes them above Atletico Madrid, fourth, while Valverde's Athletic are sixth.
"I'm annoyed because it went just past me and maybe I could have done a bit more," Simon told DAZN.
"We really wanted to get three points to make a big gap above seventh place."
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